This invention relates to toner for developing electrostatic latent images, more particularly, toner for developing electrostatic latent images used in electrophotography, electrostatic recording and electrostatic printing for formation of copied images with high quality.
In the development of electrostatic latent images in electrophotography, electrostatic recording and electrostatic printing, the electrostatic latent images formed on a photosensitive member are made visible by providing fictionally charged toner.
As a conventional method for charging toner electrically, there are known a two-component developing system in which toner is mixed and stirred with carrier to be charged electrically, and a single-component developing system in which toner is charged tribo-electrically in contact with a developing sleeve, a controlling blade, or a photosensitive member. In either method of the two, unless toner is charged uniformly, there arise disadvantages in a developing process and a transferring process.
Recently, there is provided toner of layered type in order to meet requirements for high resolving power, high quality, various functions or diverse uses.
The layered toner is formed of plural layers, each of which is given different functions so that properties required for toner such as fixing properties, coloring properties, chargeability and the like may be shown at their best.
As the fixing properties are generally given to core particles in layered toner, it is necessary that the core particle has low melting viscosity. For colored toner of layered type, the melting viscosity of the core needs to be lowered to obtain light-transmittance. On the other hand, as a coating layer plays an important role in storage properties, heat resistance and the like, it is necessary that the coating layer has high melting viscosity. Further, the core is needed to be formed of resin with high adhesive force in order to keep adhesion of the coating layer to the core material. However, when the coating layer is formed, the core material melts partially to fuse with the coating layer, with the result that the core resin is partially bared out of the surface of the toner. The partially bared resin of the core causes aggregation of toner particles in a copying machine, deterioration of storage properties of toner.
As another problem, an outermost surface layer or an intermediate layer of layered toner is liable to peel off or abrade by mixing, stirring or friction with carrier in a developing machine. If the resin with low viscosity inside the outermost surface layer appears out of the surface, it brings about changes of basic properties of toner such as heat resistance and the like.
Further, the particles which are produced by separation or abrasion influence toner or carrier each other, resulting in adverse influences on chargeability of developer and quality of copied images.
The particles peeled off from the coating layers of toner are very small, and adhered firmly to a photosensitive member. As the adhered particles are difficult to remove with a cleaner, there arise such problems as cleaning failure, filming phenomenon and the like. When fine particles (in particular, 5 .mu.m or less in particle size) increase, the flowability of a developer decreases greatly, with the result in insufficient stirring, aggregation of developer and decrease of developing efficiency.
With respect to layered toner, Japanese Patent Laid-Open No. 61-275767 discloses layered toner formed of a layer containing magnetic body and/or coloring agent on core particle in a wet process and a capsule layer prepared by polymerizing one or more monomers selected from fluorine-containing monomers, amino-containing monomers and nitro-containing monomers. Japanese Patent Published No. 59-38583 discloses toner with coating layers formed on core particles in a wet process in which the coating layers are prepared with fine particles obtained by emulsion polymerization. Japanese Patent Laid-Open No. 62-226162 discloses toner in which fine resin particles are adhered to surfaces of colored thermoplastic resin in a wet process followed by heat treatment.
Both of the techniques above mentioned utilize the dependence of electrical properties on surface portions of toner to aim to stabilize chargeability of toner by adjusting physical properties of resin of the surface layer or shapes of the surface layers. But, these resin layers adhered to surfaces of core particles by the wet process are formed of fine resin particles fixed on core particles with the shapes of particles kept as they are. Accordingly, the resin layers do not cover the surfaces of core particles completely (that is, the layers are not dense.). Therefore, toner is influenced adversely by coloring agents, magnetic particles and the like contained in core particles, with the result that the toner particles are not charged stably. In particular, toner is preserved or used under severe conditions, a resin component of the core goes outside from between fine resin particles. The bared resin influences charging stability, much more adversely, and also brings about such a problem as aggregation of toner particles.